Saturn’s largest moon holds a potentially significant surprise: a tropical lake.

A team led by University of Arizona professor Caitlin Griffith has discovered a lake along Titan’s lower latitudes—a finding that contradicts what scientists’ models have so far predicted and raises new questions about Titan’s atmosphere and surface.

PHOTO: NASA/JPL-Caltech/Space Science Institute
Saturn's largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA's Cassini spacecraft.

Like Earth, Titan has clouds, lakes and rain, but they’re made of methane instead of water. Researchers aren’t sure where the methane comes from, but have proposed methane-venting volcanoes, methane aquifers that seep the compound to the surface, and other mechanisms to explain it.

Past studies have found methane lakes on Titan’s poles. These lakes aren’t surprising, says Griffith, because Titan’s atmosphere tends to circulate methane to the moon’s polar regions, and the colder temperatures there cause the methane to condense into liquid form.

So when Griffith and colleagues began observing Titan's tropics over a year ago, they didn't expect to find lakes there. They were hoping to characterize the moon's atmosphere and how it reflects sunlight.

"We thought it was going to be pretty boring," Griffith recalls.

Using data from the Cassini–Huygens Mission, the researchers mapped Titan's tropical latitudes and their reflectivity. That's when a dark area emerged--one that could indicate liquid methane.

“I was very, very surprised,” says Griffith. “We didn't expect to find anything at all. And when you see something you don't expect, you think, 'Uh oh, maybe we did something wrong.'"

A year's worth of double-checking confirmed the dark area is a roughly 927-square-mile lake of liquid methane, the team reports in Nature today. That’s about the area of the Great Salt Lake at its lowest level, says Griffith.

The finding supports the idea that methane seeps from aquifers beneath Titan’s surface, Griffith says, because methane evaporates quickly along Titan’s tropics. The lake could not exist without a recent and regular supply of methane.

Griffith and her colleagues found no clues to how subterranean methane aquifers might have formed on Titan, but they hope future studies will fill in the rest of the puzzle.